1. Field of the Disclosure
The present disclosure relates generally to a duplexing optical scanner having an automatic document feeder (ADF), and, in particular, to a system for calibrating a stationary image capture module in the ADF.
2. Description of the Related Art
An imaging apparatus, such as a copier, scanner, or a multi-function printing device may include a scanning mechanism that operates in an ADF mode and/or a manual mode. In the manual mode, a media sheet is placed on a flatbed for scanning while in the ADF mode, media sheets are sequentially fed for scanning. In some scanning mechanisms, a duplex automatic document feeder (DADF) is employed whereby two scan modules scan both sides of a media sheet as the media sheet is fed in the ADF.
For a DADF to properly scan images, the two scan modules need to be calibrated. Typically, one of these scanning modules is located in a flatbed scanner and scans one side of a media sheet, and the other is a stationary scanner located inside of the DADF and scans the opposite side of the media sheet. A calibration target used for calibrating the scan module within the flatbed scanner is disposed therein such that contamination of the calibration target with dust is prevented or at least reduced. During calibration a scan is made of white calibration target image and typically has a green hue because the green channel on the scan module, such as a CCD scan module, has the highest sensitivity. Also the light source, reflectors, mirrors, etc., can all contribute to the colors being out of balance resulting in white not appearing to be white. In such a situation the output signals of the three uncalibrated channels, red, blue and green with the CCD scan module looking at a white calibration target would be two or more distinct bands. Calibration will adjust the three color channels with the analog front end (AFE) gain and offset settings to balance the three colors and make the target appear white. The AFE gain setting acts as a multiplier on the signal while the AFE offset acts as an addition or subtraction to move the signal up or down. A dark calibration is also done where the light source is turned off to provide a black target. Largely because of thermal drift, the light source and electronics aging, this calibration is checked and updated every so often. After calibration and adjustment of the AFE gain and offset, the output signals of the three color channels will substantially overlap one another.
Another part of scan module calibration is called shading. While the AFE gain and offset settings act on the entire color channel, shading will act on individual pixels. Shading will take each pixel and adjust it to a target. The ideal end result would be a perfectly flat signal output. There are several phenomena shading attempts to compensate for: 1) in the same CCD color channel, there will be a pixel to pixel variation in light sensitivity; 2) the edges of a scan are darker due to the vignetting effect of the lens; 3) any contamination in the optical path can partially block the light from reaching a pixel; and, 4) variation in light output across the scan line. The light source is typically a string of LEDs, and each individual LED's light output will not perfectly match the others. Shading requires a perfect calibration target in order to work properly. Any dust or scratch on the calibration target will cause those pixels to be overcorrected and a vertical band will show up on the scan of a document. In the flatbed scanner this is easy to solve with a moving scan which can be used to filter defects on the calibration target. In the ADF the CCD scan module is stationary, so no compensation is available for any defect on the target.
A calibration target for calibrating the DADF scan module is more susceptible to contaminants because it is typically located along the media path of the ADF, and, thus exposed to contamination such as from media sheets being fed through the media path. For a calibration process to be robust against dust and contamination on the calibration target, a wide area of the calibration target may need to be scanned. Any contamination on the calibration target may then be removed with a filter or averaged with other scan lines of correct data. In the flatbed scanner, this can be accomplished because the flatbed scan module is designed to translate. That is, while the flatbed scanner calibration target is stationary, the flatbed scan module can be moved to scan multiple locations on the flatbed scanner calibration target. In the DADF, however, it is generally difficult to move the DADF scan module to scan multiple locations on the calibration target because of space constraints. In some existing DADFs, stationary calibration targets are used for calibrating the DADF scan module where it scans only one unique location on the calibration target. This calibration process is susceptible to contamination and debris.
Accordingly, there is a need for an improved system for calibrating a scan module in a DADF that is more robust to dust and contamination.